Fabric: mounting a child into a View with overflow hidden and a box shadow crashes with -[NSView insertSubview:atIndex:] #3154

Environment

• react-native-macos 0.81.9, New Architecture (Fabric)
• macOS 27.0.1, Apple silicon

Description

On macOS, a Fabric View whose style clips its overflow ink (overflow: 'hidden' together with a box shadow or an outline, see styleWouldClipOverflowInk) gets a separate container view from currentContainerView. That container is allocated as RCTPlatformView, which is a plain NSView on macOS:

// React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm
RCTPlatformView *_containerView; // [macOS]
...
_containerView = [[RCTPlatformView alloc] initWithFrame:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)]; // [macOS]

mountChildComponentView:index: then sends the UIKit-style insertSubview:atIndex: to self.currentContainerView. Only RCTUIView implements that selector, so the next child mounted into such a view aborts the process:

NSInvalidArgumentException: -[NSView insertSubview:atIndex:]: unrecognized selector sent to instance
-[RCTViewComponentView mountChildComponentView:index:]  (RCTViewComponentView.mm:203)
RCTPerformMountInstructions
-[RCTMountingManager performTransaction:]

main still allocates RCTPlatformView here (packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm).

Steps to reproduce

1. Render a View with overflow: 'hidden' and a boxShadow (or an outline).
2. Mount a child into it after the container exists, for example by toggling a child on and off.
3. The app aborts with the exception above.

We hit it through our app (a mail composer closing or sending) rather than a minimal project, so these steps are derived from the code path.

Suggested fix

Allocate the container as RCTUIView, which is what the currentContainerView getter already declares as its return type:

-  RCTPlatformView *_containerView; // [macOS]
+  RCTUIView *_containerView; // [macOS]
...
-      _containerView = [[RCTPlatformView alloc] initWithFrame:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)]; // [macOS]
+      _containerView = [[RCTUIView alloc] initWithFrame:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)]; // [macOS]
...
       _containerView.clipsToBounds = self.clipsToBounds;
+      _containerView.layer.masksToBounds = self.clipsToBounds;

The explicit layer assignment preserves overflow clipping: RCTUIView synthesizes clipsToBounds without forwarding it to AppKit or updating masksToBounds.

We ship this as a pnpm patch of 0.81.9. With it, an XCUITest journey that opens and closes our composer 21 times in one process keeps the app running.